The present invention relates to the field of film-type electronic devices and more particularly to film-type resistors and methods of manufacturing same.
Film-type electronic devices, and particularly film-type resistors, have been generally known, but efficient and effective methods of manufacturing have long been sought after. An early method required the spraying of the film material onto the substrate and then removing portions by cutting or grinding. Alternatively, a method was devised to draw a pattern on the substrate using an applicator with liquid film (U.S. Pat. No. 1,857,769). More recently, apparatus was developed to allow silk-screen printing of the desired pattern directly onto a cylindrical substrate (U.S. Pat. No. 3,880,609). This system suffers from the fact that expensive and elaborate equipment is needed for the operation.
Production of film-type electronic devices becomes even more difficult when the substrate required to carry the device is curved or irregular in shape. Since flat substrates are either impractical or undesirable in many applications, curved or contoured substrate surfaces are being encountered most often. With film-type resistors it is common to use a cylindrical substrate due to its superior thermal and physical shock properties. The silk-screening process described in U.S. Pat. No. 3,880,609 was designed for use with cylindrical substrates ground smooth with centerless grinding techniques, and is not possible when substrate surfaces are not ground sufficiently smooth and parallel to operate with the printing equipment. Accordingly, this prior process requires precision preparation of the substrates and careful quality control to assure adequate print quality.
Early manufacturing process (U.S. Pat. No. 3,266,661) for the production of printed circuits employed decalcomania paper upon which a circuit was silk-screened and subsequently transferred to the printed circuit board. Although presenting an attractive alternative to scribing or printing directly onto the substrate, this process was never perfected for film resistors due to the inherent disintegration of the printed resistive ink pattern during drying.